Call it a detective story for the ages: Where did gold, platinum, and other heavy elements come from?
The theory was that these metals were produced by fusing lighter elements together, but scientists determined that such a process would require staggering amounts of energy. What provided all that juice?
On Monday, an international team of more than 1,000 scientists announced the answer: fiery collisions of neutron stars.
Scientists have suspected for several decades that such collisions were responsible, but they had no direct evidence until now.
Among the team members was Amber Stuver, an assistant professor of physics at Villanova University. Her responsibilities included helping to ensure the quality of the data — measurements of "gravitational waves" that resulted from a collision of two neutron stars 130 million years ago. Scientists at Pennsylvania State University, where Stuver earned her Ph.D. in 2006, also played a key role.
In addition to wrangling the data, Stuver was assigned to a group tasked with communicating the findings to the public, so we asked her for the backstory to this celestial mystery. Here's the abbreviated version:
Remember in high-school physics class where you learned that the sun is powered by fusion? Hydrogen is fused together in a series of reactions that yield helium, releasing the solar energy that we see and feel here on earth.
Heavier elements can be made through fusion as well. Yet for elements heavier than iron, their formation does not produce energy, but requires the addition of more.
"Iron is the tipping point," Stuver said. "In order to make the heavier elements, you would have to have some way of smashing their nuclei together."
Scientists have determined that some of the energy needed for this process could come from supernovas — large explosions that take place at the end of a star's life cycle.
But there has not been enough energy from supernovas to explain all of the known gold, platinum, and other heavy elements in the solar system.
"We basically have an excess of gold," Stuver said.
So the scientists searched for signs of collisions between ultra-dense neutron stars, and on Monday they reported the first evidence of such an event. Measurements revealed gravitational ripples from the violent explosion eons ago, the team reported in a series of papers.
The team also detected "absorption lines" — signatures of certain heavy elements, called lanthanides, that were created in the same event. Stuver cautioned that the team did not detect signatures for gold and platinum, but said the case was strong that these precious elements also had been created.
"We found evidence that gold should have been made because we did find other elements that are made in the same way," she said.